Non-destructive testing of large diameter steel pipes, such as natural gas pipelines, using magnetic-inspection techniques is well known in the art. Tools fitted with magnetic flux leakage (MFL) anomaly detectors embodying strong permanent magnets to magnetize the pipe wall to near saturation flux density are generally employed. Sensors, moving with the detectors, have signals induced by variations in the leakage fields and fluxes caused by such pipe-wall anomalies as internal or external corrosion, hard spots and so on, and including local variations in the magnetic permeability, often induced by local residual or stress concentrations. MFL anomaly detectors are, however, subject to problems caused by such things as noise, hostile operating environment, restricted power due to battery capacity and finite data storage capacity. Thick walled or small diameter pipes are more difficult to inspect by the MFL anomaly technique because there is frequently little space available inside such pipes for magnets compared to the relatively large pipe-wall cross sectional area to be driven into magnetic saturation. While this problem has been alleviated in recent years following the introduction of neodymium-iron-boron (NdFeB) permanent magnets, which have improved mechanical and magnetic properties and can produce higher usable flux densities than the earlier ferrite magnets and are mechanically stronger than cobalt-rare earth magnets, MFL anomaly techniques are generally limited to essentially direct contact inspection and are not generally suitable for indirect coupling through air or other materials or when there is appreciable sensor lift-off. Remote field eddy-current (RFEC) devices have been developed for these latter tasks.
RFEC devices incorporate transmit and receive coils and commonly use a solenoidal exciter coil, energized with low frequency ac or pulsed current (typically, 20–2000 Hertz), internal to and generally approximately coaxial with the longitudinal axis of the pipe to be tested. It has been found that the appropriate operating frequencies for non-ferromagnetic tubes, such as reactor pressure tubes are much higher, at about 10 kHz, than those suitable for winding ferromagnetic pipes, which are typically about, though preferably not at, 60 Hz. Low frequencies imply low scanning speeds. The detector coil, or array of detector coils, is placed adjacent the inside of the pipe wall and may be axially, radially or even circumferentially aligned. The exciter coil or coils are normally displaced longitudinally along the pipe from the detector coil by customarily about 2 to 3 pipe diameters (D). At this separation direct coupling inside the pipe between the exciter and detector is strongly attenuated by the eddy currents induced in the conducting pipe. The signal in the detector results principally from an indirect energy transmission path on the outside of the pipe. Field from the exciter diffuses through the pipe wall in the vicinity of the exciter, being attenuated and phase shifted in the process. Once on the outside, this energy radiates with relatively little attenuation. In the case of a ferromagnetic pipe, it tends to be guided preferentially in the axial direction.
Adjacent the remote detector coil or coils, the field on the outside of the pipe is greater than the field inside, which is generated largely by energy which diffuses back from the outside, again being attenuated and further phase shifted in the process. Anomalies anywhere in this through wall indirect energy transmission path will cause changes in the phase and amplitude of the received signals. Because the received signals are small, typically of the order of 10 μv a phase sensitive synchronous detector or locked-in amplifier is incorporated to receive and amplify the signal.
While RFEC probes have now been used for many years for well casing inspection and more recently for heat exchangers and pressure tubes, the phenomenon is complex and defect responses are still not fully understood. Large diameter prestressed concrete pressure pipes (PCPP) have been used to convey water for many years and such pipes are frequently provided with a spirally wound high strength prestressing wire which prestresses the concrete before a top coating of mortar is applied. There are many types of PCPP. The cores of some may be just high strength concrete and are generally described as “no-cylinder” pipe, others, such as lined cylinder pipe (LCP) may have a thin metallic cylinder, preferably steel, or other metal such as iron or a nickel alloy, with a concrete core, often centrifugally cast, inside. Still others, such as the typically larger diameter embedded cylinder pipe (ECP), have an additional layer of concrete applied to the outside of the steel or alloy pipe before the prestressing rod, bar or wire is wound and a protective layer of mortar is impacted on top of the rod, bar or wire spiral. Some types of PCPP may be provided with both axially spaced reinforcing rods and spirally wound circumferential wires. As used in this specification, all of the above types of prestressed concrete pressure pipes are included within the definition “PCPP”. Although generally high strength cold drawn steel prestressing wire is used, relatively lower strength hot or cold rolled bar or rod can also be used in some applications and these PCCP are generally described as bar wrapped. As used in this specification the term “prestressing wire” is to be construed to include both cold drawn wire and hot or cold rolled bar or rod. The surface of the prestressing wire and also the steel cylinder are often coated prior to winding with a highly alkaline slurry designed to promote the formation of a protective oxide film that will tend to inhibit corrosion of the steel.
While rupture of PCCP is relatively uncommon, nevertheless periodic inspection of water supply lines and the like, which have an expected service life of 50 years or more, would be advantageous in order to prevent expensive ruptures or other failures. Prior to my aforesaid earlier patent on the RFEC/TC, often loosely referred to as the “remote field” technique, there was no practical method for inspecting composite pipes, such as PCPP, and my earlier methods are susceptible to improvements.